There is a need for core-shell emulsion polymers with a core or rubbery stage based on homopolymers or copolymers of butadiene for use as impact modifiers in matrix polymers such as acrylonitrile-butadiene-styrene ("ABS"), for styrene-acrylonitrile copolymers, in methyl methacrylate polymers, in poly(vinyl chloride) ("PVC"), in various engineering resins such as polycarbonate, polyesters, or polyamides, and in thermosetting resins such as epoxies. Such impact modifiers containing copolymers of butadiene and styrene and at least one stage or shell of poly(methyl methacrylate) are known in the art as methacrylate-butadiene-styrene ("MBS") core-shell polymers.
Although MBS impact modifiers are commercially available, there is a constant need to reduce the cost of manufacturing these impact modifiers while maintaining or improving the properties of the matrix resins that they are used in. Improved properties may include enhanced impact properties (e.g., PVC bottles that can withstand higher drop heights without breaking) and improved clarity and color in clear matrix polymer blends. Reducing manufacturing costs may arise from shorter process times and/or reducing the cost of powder recovery processes. It is desirable to use efficient emulsion recovery processes known in the art, such as spray-drying or coagulation. Coagulated MBS emulsions produce a slurry of coagulated core-shell polymer particles of varying particle size which must be further dried to produce MBS powders which are readily handled and added to matrix resins during blending and compounding processes. Commercially-viable average slurry particle sizes for generating powders are typically in the range of from about 100 microns to less than about 300 microns, and preferably between about 200 and 250 microns. A preferable average slurry particle size is needed to avoid problems related to dispersion in matrix polymers, formation of dust when handled, powder compaction, flowability, and wetcake drying.
U.S. Pat. No. 5,534,594, Troy et al. discloses an improved process for preparing core-shell MBS modifiers wherein feeds of the monomers which form the second and/or third stage polymers are added prior to completion of polymerization of the earlier stages. Although the Troy process substantially reduces polymerization time, I discovered that commercially viable latex core-shell polymer emulsions, having a butadiene-based core weight fraction of greater than about 70% and a first stage core monomer conversion less than 75%, provide slurries undesirably having an average particle size greater than about 300 microns when coagulated at temperatures greater than about 20.degree. C. Because slurry particle size typically increases with increasing temperature, commercially-viable impact modifiers which follow the Troy process require coagulation temperatures below 20.degree. C. in order to provide slurries having a desirable average particle size less than about 300 microns.
Coagulating below about 20.degree. C. is a problem as it precludes the use of efficient commercial-scale cooling processes. While Troy discloses coagulation ("Method B") as a possible isolation method, Troy is not concerned with maintaining a desirable average slurry particle size. When core-shell polymers made according to Troy are coagulated at temperatures at or above about 20.degree. C., the average slurry particle size frequently exceeds about 300 microns. Moreover, the "Method A" isolation process described by Troy (freeze drying of emulsion followed by vacuum drying) is not amenable for commercial production.
The ability of a particular MBS core-shell polymer to increase the impact strength of a matrix polymer generally increases, up to a point, as the rubbery core weight fraction increases, generally to at least 70%. However, as the core weight fraction increases there is a corresponding decrease in the weight fraction, thickness, and hardness of the outer polymer shell. If the shell becomes too thin, it will not properly cover the rubber core.
Improper shell coverage also arises as described in Makromol. Chem., Macromol. Symp. 35136, 307-325 (1990). Improper shell coverage in MBS polymers is also exacerbated by the presence of unreacted monomer in the shell, which reduces the glass transition ("Tg") of the shell. Ultimately, improper shell coverage leads to problems with powder isolation (e.g., inability to spray dry or coagulate, or very large slurry particle size), and reduced impact strength in matrix polymer blends. These problems usually become apparent when the rubbery core weight fraction exceeds about 70% to 75%.
I have discovered that a certain amount of an alkyl acrylate and/or crosslinking monomer polymerized between the inner graft stage and the outer shell decreases the average slurry particle size of coagulated multistage MBS core-shell polymers having a rubbery core weight fraction exceeding about 70% to a commercially-viable range (less than about 300 microns at coagulation temperatures above 20.degree. C.).
I have also discovered that core-shell polymers prepared at reaction temperatures in the range of from 60.degree. C. to 70.degree. C. provide a suprisingly large improvement in impact properties in matrix polymer blends.
The object of the present invention is to provide MBS core-shell polymers having a rubbery core weight fraction exceeding about 70% and a slurry particle size smaller than 300 microns when coagulated above 20.degree. C. for use as impact modifiers in matrix resins. A further object is to provide an improved impact modifier having a rubbery core weight fraction exceeding about 70% and a slurry particle size smaller than 300 microns when coagulated above 20.degree. C. for optically clear PVC compositions. Another object of the invention is to provide thermoplastic blends and articles comprising matrix polymers and the MBS core-shell polymers of this invention having a suprisingly large improvement in impact properties. A still further object is to provide a process for preparing core-shell MBS impact modifiers having a rubbery core weight fraction exceeding about 70%, a slurry particle size smaller than 300 microns when coagulated above 20.degree. C., and a suprisingly large improvement in impact properties when blended with matrix resins. These and other objects as will become apparent from the following disclosure are achieved by the present invention.